Cold-rolled steel sheet and plated steel sheet

ABSTRACT

A cold-rolled steel sheet includes, by mass %: C: 0.020% or more and 0.080% or less; Si: 0.20% or more and 1.00% or less; Mn: 0.80% or more and 2.30% or less; and Al: 0.010% or more and 0.100% or less; and further includes: one or more of Nb and Ti which satisfy a requirement of 0.005%≤Nb+Ti&lt;0.030%, in which a structure consists of, ferrite, bainite, and other phases, an area ratio of the ferrite is 80% or more and less than 95%, an area ratio of a non-recrystallization ferrite in the ferrite is 1% or more and less than 10%, an area ratio of the bainite is 5% or more and 20% or less, a total amount of the other phases is less than 8%, an equivalent circle diameter of a carbonitride including one or both of Nb and Ti is 1 nm or more and 10 nm or less, and a tensile strength is 590 MPa or more.

This application is a Divisional of U.S. application Ser. No.14/373,748, filed Jul. 22, 2014, which is the U.S. National Phase ofPCT/JP2013/052762, filed Feb. 6, 2013, which claims priority under 35U.S.C. 119(a) to Japanese Patent Application No. 2012-028271, filed Feb.13, 2012, the contents of all of which are incorporated by reference, intheir entirety, into the present application.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a high-strength cold-rolled steel sheetand a plated steel sheet, which have excellent ductility and stretchflangeability and are suitable for an automobile steel sheet,particularly suitable for a structural member (for example, a bracket),and a method for manufacturing the same.

Priority is claimed on Japanese Patent Application No. 2012-028271,filed on Feb. 13, 2012, the content of which is incorporated herein byreference.

RELATED ART

In recent years, in order for automakers to cope with the tightening ofCO₂ emission regulations in Europe in 2012, fuel economy regulations inJapan in 2015, and stricter collision regulations in Europe,high-strengthening of steel to be used has rapidly progressed to improvefuel economy through a decrease in the weight of an automobile body andimprove collision safety. Such a high-strength steel sheet is called a“high tensile strength steel sheet”, and the amount of orders of thinsteel sheets mainly having a tensile strength of 440 MPa to 590 MPa, andrecently more than 590 MPa, tends to increase every year. Among them,excellent ductility and stretch flangeability are demanded for astructural member such as a bracket in view of the working method.Generally, it is considered that when the product of tensile strengthand total elongation is 17000 MPa·% or more, ductility is excellent,and, regarding a grade of 590 MPa of tensile strength, when holeexpansion ratio is 80% or more, stretch flangeability is excellent.

Generally, when tensile strength increases, yield strength alsoincreases. Thus, ductility is decreased, and further, stretch flangeformability is deteriorated. In the related art, in a case of dual phase(DP) steel including two phases of ferrite and martensite, the ductilityis excellent, but micro-cracks caused by local strain concentration inthe vicinity of a boundary between ferrite which is a soft phase andmartensite which is a hard phase easily occur and propagate, and thus,it is considered that the dual phase is a disadvantageous microstructurein hole expansibility. Accordingly, it is considered that the smallerthe hardness difference between the microstructures is, the moreadvantageous it is in hole expansibility improvement, and thus, a steelsheet having a uniform structure such as ferrite single phase steel orbainite single phase steel is considered to be superior. From the aboveviewpoint, it is important to control a constituent phase fractionmatched with a desired tensile strength to attain both ductility andhole expansibility.

As a high-strength steel sheet in which both of ductility and stretchflangeability are attained, a steel sheet in which precipitationstrengthening is actively utilized has been proposed so far (forexample, refer to Patent Documents 1 and 2).

However, since the cold-rolled steel sheet proposed in Patent Document 1is mostly annealed almost within a ferrite single phase region,structure strengthening by bainite is hardly utilized. Thus, in order tofacilitate high-strengthening, a large amount of Ti and precipitationelements other than Ti have to be added to actively utilizeprecipitation strengthening. Subsequently, a higher alloy cost isrequired. In addition, precipitation elements such as Ti and Nb alsofunction as recrystallization inhibiting elements, and thus, when theseelements are added in a large amount, recrystallization is remarkablydelayed in annealing. Accordingly, in order to have the area ratio ofnon-recrystallization ferrite of 25% or less, it is assumed that atemperature rising rate needs to become extremely slow in the annealingstep or that a holding time at the maximum heating temperature needs tobecome extremely long, and thus, productivity is deteriorated. Inaddition, since precipitation strengthening is actively utilized in acold-rolled steel sheet proposed in Patent Document 2 as in PatentDocument 1, a large amount of Ti and precipitation elements other thanTi have to be added to the cold-rolled steel sheet proposed in PatentDocument 2. Subsequently, a higher alloy cost is required and also whenthese elements are added in a large amount, recrystallization isremarkably delayed in annealing. Thus, in order to have the area ratioof non-recrystallization ferrite of 25% or less, the maximum heatingtemperature in the annealing step becomes extremely high. Alternatively,when the maximum heating temperature is just higher than an Ac₁transformation temperature, a temperature rising rate becomes extremelyslow. Alternatively, it is assumed that a holding time at the maximumheating temperature needs to become extremely long, and thus,productivity is deteriorated.

In addition, a steel sheet having improved stretch flangeability byactively utilizing non-recrystallization ferrite to reduce the hardnessdifference between ferrite and a hard phase has been proposed (forexample, refer to Patent Documents 3 to 5).

However, since it is necessary to add a large amount ofrecrystallization inhibiting elements such as Nb and Ti to activelyutilize non-recrystallization ferrite, a higher alloy cost is requiredand also a temperature rising rate needs to be increased in an annealingstep. Thus, facility investment is needed.

PRIOR ART DOCUMENT Patent Document

-   [Patent Document 1] Japanese Unexamined Patent Application, First    Publication No. 2010-285656-   [Patent Document 2] Japanese Unexamined Patent Application, First    Publication No. 2010-285657-   [Patent Document 3] Japanese Unexamined Patent Application, First    Publication No. 2008-106352-   [Patent Document 4] Japanese Unexamined Patent Application, First    Publication No. 2008-190032-   [Patent Document 5] Japanese Unexamined Patent Application, First    Publication No. 2009-114523

DISCLOSURE OF THE INVENTION Technical Problem

The present invention is to stably provide a high-strength cold-rolledsteel sheet and a plated steel sheet which have excellent ductility andstretch flangeability, without deterioration in productivity.

Solution to Problem

The present invention is a finding obtained from an investigation thathas been conducted to solve the above mentioned problems of improvingductility and stretch flangeability of a high-strength cold-rolled steelsheet, a hot-dip galvanized steel sheet, and a galvannealed steel sheetwhich have a tensile strength of 590 MPa or more. That is, anappropriate microstructure is attained by optimizing the amount ofalloying elements, particularly, optimizing the amount of Nb, and Ti andpositively adding Si. In addition, in an annealing process, the maximumheating temperature is controlled within a temperature range of Ac₁ [°C.] or more and (Ac₁+40) [° C.] or less and an end temperature and acooling rate of a primary cooling after annealing are determined.Accordingly, a sufficient recrystallization suppressing effect can beobtained, and thus, while utilizing bainite, the amount ofnon-recrystallization ferrite is appropriately controlled by controllingan equivalent circle diameter of the carbonitrides including one or bothof Nb and Ti to fine. The present invention is made based on thefindings that it is possible to produce a steel sheet having excellentductility and stretch flangeability compared to steel sheets of therelated art, and the summary thereof is described as follows.

(1) According to a first aspect of the present invention, there isprovided a cold-rolled steel sheet including, by mass %: C: 0.020% ormore and 0.080% or less; Si: 0.20% or more and 1.00% or less; Mn: 0.80%or more and 2.30% or less; P: 0.0050% or more and 0.1500% or less; S:0.0020% or more and 0.0150% or less; Al: 0.010% or more and 0.100% orless; N: 0.0010% or more and 0.0100% or less; and further including: oneor more of Nb and Ti which satisfy a requirement of 0.005%<Nb+Ti<0.030%;and a reminder including Fe and unavoidable impurities, in which astructure consists of, a ferrite, a bainite, and an other phase, theother phase includes a pearlite, a residual austenite, and a martensite,an area ratio of the ferrite is 80% or more and less than 95%, an arearatio of a non-recrystallization ferrite in the ferrite is 1% or moreand less than 10%, an area ratio of the bainite is 5% to 20%, a totalamount of the other phase is less than 8%, an equivalent circle diameterof a carbonitride including one or both of Nb and Ti is 1 nm or more and10 nm or less, and a tensile strength is 590 MPa or more.

(2) The cold-rolled steel sheet according to (1) may further include oneor more of, by mass %: Mo: 0.005% or more and 1.000% or less; W: 0.005%or more and 1.000% or less; V: 0.005% or more and 1.000% or less; B:0.0005% or more and 0.0100% or less; Ni: 0.05% or more and 1.50% orless; Cu: 0.05% or more and 1.50% or less; and Cr: 0.05% or more and1.50% or less.

(3) According to a second aspect of the present invention, there isprovided a plated steel sheet in which plating is provided on a surfaceof the cold-rolled steel sheet according to (1) or (2).

(4) According to a third aspect of the present invention, there isprovided a method for manufacturing a cold-rolled steel sheet including:heating a slab having a chemical composition according to (1) or (2) to1150° C. or more and 1280° C. or less; finishing a finish rolling undera temperature of Ar₃° C. or more and 1050° C. or less; pickling and thencold-rolling a hot-rolled steel sheet, which is coiled under atemperature range of 450° C. or more and 650° C. or less, under areduction of 40% or more and 70% or less; thereafter heating into atemperature range of Ac₁° C. or more and (Ac₁+40) ° C. or less under arate of 2° C./sec or more and 5° C./sec or less; annealing thecold-rolled steel sheet under the temperature range of Ac₁° C. or moreand (Ac₁+40) ° C. or less and for a holding time of 10 sec or more and200 sec or less; and primary cooling immediately after the annealinginto a steel sheet temperature range of 600° C. or more and 720° C. orless under a cooling rate of 10° C./sec or less in a course from theannealing to arriving at a normal temperature, in which the Ar₃° C. andthe Ac₁° C. is a Ar₃ transformation temperature and a Ac₁ transformationtemperature, respectively, calculated from Expressions 1 and 2,

Ar₃=910−325×[C]+33×[Si]+287×[P]+40×[Al]−92×([Mn]+[Mo]+[Cu])−46×([Cr]+[Ni])  (Expression1),

Ac₁=723+212×[C]−10.7×[Mn]+29.1×[Si]  (Expression 2), and

elements noted in brackets represents an amount of the elements by mass%, respectively.

(5) According to a fourth aspect of the present invention, there isprovided a method for manufacturing a plated steel sheet includingplating the cold-rolled steel sheet manufactured by the method accordingto (4) after the annealing and the cooling.

(6) The method for manufacturing a plated steel sheet according to (5)may further include heat treating the plated steel sheet under atemperature range of 450° C. or more and 600° C. or less with 10 secondsor longer.

Effects of the Invention

According to the present invention, it is possible to provide ahigh-strength cold-rolled steel sheet, a hot-dip galvanized steel sheet,and a galvannealed steel sheet which have a tensile strength of 590 MPaor more, and excellent ductility and stretch flangeability. Therefore,the present invention makes an extremely significant contribution to theindustry.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a graph showing a relationship between the maximum heatingtemperature, particularly, an Ac₁ transformation point or higher, inannealing and an area ratio of non-recrystallization ferrite.

FIG. 2 is a graph showing a relationship between an area ratio ofnon-recrystallization ferrite and a hole expansion ratio 7.

FIG. 3 is a graph showing a relationship between the maximum heatingtemperature, particularly, an Ac₁ transformation point or higher, inannealing and an area ratio of bainite.

FIG. 4 is a graph showing a relationship between an area ratio ofbainite and a hole expansion ratio X.

FIG. 5 is a graph showing a relationship between the maximum heatingtemperature, particularly, an Ac₁ transformation point or higher, inannealing and an equivalent circle diameter of carbonitrides.

FIG. 6 is a graph showing a relationship between an equivalent circlediameter of carbonitrides and an area ratio of non-recrystallizationferrite.

FIG. 7 is a graph showing a relationship between a total amount of otherphase and a hole expansion ratio λ.

EMBODIMENTS OF THE INVENTION

Hereinafter, the present invention will be described in detail.

First, the reasons why steel components are limited in the presentinvention will be described.

C is an element which contributes to an increase in tensile strength andyield strength, and added in appropriated amounts according to atargeted strength level. In addition, C is also effective in obtainingbainite. When the amount of C is less than 0.020%, it is difficult toobtain a target tensile strength and yield strength, and thus, the lowerlimit is set to 0.020%. On the other hand, when the amount of C is morethan 0.080%, deterioration in the ductility, hole expansibility, andweldability is caused. Thus, the upper limit is set to 0.080%. Inaddition, in order to stably secure the tensile strength and yieldstrength, the lower limit of C may be preferably 0.030% or 0.040%, andthe upper limit of C may be preferably 0.070% or 0.060%.

Si is an extremely important element in the present invention. Si iseffective in improving the stretch flangeability by hardening ferritethrough solid solution strengthening to reduce the hardness differencewith a hard phase. In order to obtain the effect, it is necessary to setthe amount of Si to 0.20% or more, and thus, the lower limit is set to0.20%. On the other hand, Si may cause a problem of a decrease inplating wettability when hot dip galvanizing is carried out and aproblem of a decrease in productivity due to the delay of alloyingreaction. Therefore, the upper limit of the amount of Si is set to1.00%. In addition, Si is a ferrite stabilizing element. The lower limitof Si may be set to 0.30% or 0.40% and the upper limit of Si may be setto 0.90% or 0.80% to obtain an appropriate amount of bainite.

Mn acts as an element that contributes to solid solution strengthening,and thus has an effect of increasing the strength. In addition, Mn iseffective in obtaining beinite. In addition, it is necessary to contain0.80% or more of Mn to improve hole expansibility. On the other hand,when the amount of Mn is more than 2.30%, deterioration in holeexpansibility and weldability is caused, and thus, the upper limitthereof is set to 2.30%. In addition, in order to stably obtain bainite,the lower limit of Mn may be set to 1.00%, 1.20%, or 1.80%, and theupper limit of Mn may be set to 2.10%, or 2.00%.

P is an impurity, and is segregated at grain boundaries to cause adecrease in the toughness of the steel sheet and deterioration in theweldability. Further, the alloying reaction becomes extremely slowduring hot dip galvanizing, and the productivity is degraded. From theviewpoints, the upper limit of the amount of P is set to 0.1500%. SinceP is an element which increases strength at a low price, the lower limitof the amount of P is preferably set to 0.0050% or more. In order tofurther improve the toughness and the weldability, the lower limit of Pmay be set to 0.0060% or 0.0070%, and the upper limit of P may be set to0.1000% or 0.0850%.

S is an impurity and when the content thereof is more than 0.0150%, hotcracking is induced or workability is deteriorated. Thus, the upperlimit of the amount of S is set to 0.0150%. Due to restriction onproduction costs, the lower limit of the amount of S is set to 0.0020%.In order to improve the workability, the lower limit of S may be set to0.0025%, and the upper limit of S may be set to 0.0100% or 0.0080%.

Al is a ferrite stabilizing element similar to Si. Al is a deoxidizingelement and the lower limit is set to 0.010% or more in view ofdeoxidation. In addition, excessive addition of Al causes deteriorationin the weldability, and thus, the upper limit thereof is set to 0.100%.The lower limit of Al may be set to 0.015% or 0.025%, and the upperlimit of Al may be set to 0.080%, 0.060% or 0.040%.

N is an impurity. When the amount of N is more than 0.0100%,deterioration in toughness and ductility and occurrence of cracking in aslab are significant. Since N is effective in increasing tensilestrength and yield strength, similar to C, N may be positively added asthe upper limit of the amount of N is set to 0.0100%. In addition, N iseffective in obtaining bainite. Due to restriction on production costs,the lower limit of the amount of N is set to 0.0010%. The lower limit ofN may be set to 0.0020% or 0.0030%, and the upper limit of N may be0.0080%, 0.0060%, or 0.0050%.

Further, Nb and Ti are extremely important elements in the presentinvention. These elements have an effect of delaying the progress ofrecrystallization in an annealing process to allow non-recrystallizationferrite to remain. Since the non-recrystallization ferrite contributesto hardening of ferrite, the amount of the non-recrystallization ferriteis appropriately controlled to reducing the hardness difference betweenferrite and the hard phase, thereby obtaining an effect of improving thestretch flangeability. When one or more of Nb and Ti are contained so asto satisfy the condition of 0.005%≤Nb+Ti<0.030%, the reason why theupper limit of at least one of Nb and Ti is less than 0.030% is thatwhen one or more of Nb and Ti are added at a content more than the upperlimit, the non-recrystallization ferrite remains excessively, and theductility is decreased. In addition, the reason why the lower limit ofone or more of Nb and Ti are set to 0.005% is that when one or more ofNb and Ti are added at a content less than the lower limit, arecrystallization suppressing effect is small, and thenon-recrystallization ferrite hardly remains. In addition, in order toimprove the stretch flangeability, the lower limit of one or more of Nband Ti may be set to 0.010%, and the upper limit of one or more of Nband Ti may be set to 0.025%.

All of Mo, W, and V are recrystallization inhibiting elements, and oneor more of these elements may be added as required. In order to obtainthe effect of strength improvement, 0.005% of Mo, 0.005% of W, and0.005% of V are preferably added respectively as the lower limits. Onthe other hand, since excessive addition causes an increase in an alloycost, the upper limits are preferably set to 1.000% of Mo, 1.000% of W,and 1.000% of V, respectively.

All of B, Ni, Cu, and Cr are elements which increase hardenability, andone or more of these elements may be added as required. In order toobtain the effect of strength improvement, 0.0005% of B, 0.05% of Ni,0.05% of Cu, and 0.05% of Cr are preferably added respectively as thelower limits. On the other hand, since excessive addition causes anincrease in an alloy cost, the upper limits are preferably set to0.0100% of B, 1.50% of Ni, 1.50% of Cu, and 1.50% of Cr, respectively.

The high-strength cold-rolled steel sheet containing the above-describedchemical composition may contain impurities unavoidably incorporated ina production process within the range in which a reminder including ironas a main component does not impair the properties of the presentinvention.

Next, the reasons why a production method is limited will be described.

A slab having the above-described composition is heated at a temperatureof 1150° C. or higher. The slab may be a slab immediately after beingproduced by a continuous casting facility or a slab produced by anelectric furnace. The reason why the temperature is limited to 1150° C.or higher is to sufficiently decompose and dissolve carbonitride formingelements and carbon in the steel. In order to dissolve the precipitatedcarbonitrides, the temperature is preferably 1200° C. or higher.However, when the heating temperature is higher than 1280° C., thetemperature is not preferable in view of production costs, and thus,1280° C. is preferably set as the upper limit.

When a finishing temperature in hot rolling is lower than an Ar₃transformation temperature, carbonitrides are precipitated and theparticle size is coarsened in the surface area, and the tensile strengthand stretch flangeability are decreased after the annealing, so that theAr₃ transformation temperature is set as the lower limit. A temperatureof 900° C. or higher is preferable to stably precipitate theprecipitates of carbonitrides with an equivalent circle diameter of 10nm or less. The upper limit of the finishing temperature issubstantially 1050° C. in view of the slab heating temperature.

Here, Ar₃° C. is an Ar₃ transformation temperature obtained by thefollowing Expression 1.

Ar₃=910−325×[C]+33×[Si]+287×[P]+40×[Al]−92×([Mn]+[Mo]+[Cu])−46×([Cr]+[Ni])  (Expression1)

Wherein, elements noted in brackets represent an amount of the elementsby mass %, respectively.

A coiling temperature after finishing rolling is an extremely importantproduction condition in the present invention. In the present invention,the suppression of the precipitation of carbonitrides at the stage ofthe hot-rolled steel sheet with setting the coiling temperature to 650°C. or lower is important, and the properties of the present invention isnot deteriorated by the history up to that time. When the coilingtemperature is higher than 650° C., carbonitrides are precipitated andcoarsened in the hot-rolled steel sheet, sufficient recrystallizationsuppressing effects cannot be attained during annealing, and thus, 650°C. is set as the upper limit. Further, when the coiling temperature islower than 450° C., the strength of the hot-rolled steel sheet isincreased and rolling load is increased during cold rolling. Therefore,450° C. is set as the lower limit.

Cold rolling after typical pickling is carried out under a reduction of40% to 70%. When the reduction is less than 40%, the driving force ofrecrystallization becomes small during the annealing, and thus,non-recrystallization ferrite remains excessively after the annealing,which causes a decrease in the ductility. Thus, the lower limit is setto 40%. In addition, when the reduction is more than 70%, the drivingforce of recrystallization becomes large during the annealing, and thus,a small amount of non-recrystallization ferrite remains, which causes adecrease in the tensile strength and the stretch flangeability.Therefore, the upper limit is set to 70%.

The annealing is preferably carried out by the continuous annealingfacility to control the heating temperature and the heating time. Themaximum heating temperature in the annealing is an extremely importantproduction condition in the present invention. The lower limit of themaximum heating temperature is set to an Ac₁ transformation temperature,and the upper limit is set to (Ac₁ transformation temperature+40°) C.When the maximum heating temperature is lower than the Ac₁transformation temperature, a sufficient amount of a hard phase andnon-recrystallization ferrite are not obtained, and a decrease in thetensile strength is caused. On the other hand, when the maximum heatingtemperature is higher than (Ac₁ transformation temperature+40°) C., theamount of the non-recrystallization ferrite is reduced as shown in FIG.1, and thus, the stretch flangeability is decreased as shown in FIG. 2.The amount of bainite is increased as shown in FIG. 3, and thus, thestretch flangeability is decreased as shown in FIG. 4. Since thecarbonitrides are coarsened as shown in FIG. 5, the amount of thenon-recrystallization ferrite is reduced as shown in FIG. 6, and thestretch flangeability is decreased as shown in FIG. 2. Therefore, (Ac₁transformation temperature+40)° C. is set as the upper limit.

Here, Ac₁° C. is an Ac₁ transformation temperature obtained by thefollowing Expression 2.

Ac₁=723+212×[C]−10.7×[Mn]+29.1×[Si]  (Expression 2)

Wherein, elements noted in brackets represent an amount of the elementsby mass %, respectively.

A temperature rising rate is set to 2° C./sec to 5° C./sec in theannealing. When the temperature rising rate is less than 2° C./sec, notonly is the productivity deteriorated, but also recrystallizationsubstantially proceeds to reduce the amount of non-recrystallizationferrite, and thus, the tensile strength and the stretch flangeabilityare decreased. Therefore, the lower limit is set to 2° C./sec. Inaddition, when the temperature rising rate is more than 5° C./sec,non-recrystallization ferrite remains excessively, and the ductility isdecreased. Thus, the upper limit is set to 5° C./sec.

A holding time at the maximum heating temperature in the annealing is anextremely important production condition in the present invention. Theholding time of the steel sheet within the temperature range of the Ac₁transformation temperature to (Ac₁ transformation temperature+40) ° C.is set to 10 seconds to 200 seconds. This is because when the holdingtime of the steel sheet at the maximum heating temperature is shorterthan 10 seconds, non-recrystallization ferrite remains excessively, andthus, the ductility is decreased. On the other hand, when the holdingtime of the steel sheet at the maximum heating temperature is increased,a decreased in the productivity is caused and also the amount ofnon-recrystallization ferrite is reduced. Then, the tensile strength andthe stretch flangeability are decreased, and thus, the upper limit isset to 200 seconds.

In addition, after the annealing, primary cooling for cooling the steelwithin a steel sheet temperature range of 600° C. to 720° C. may becarried out under a cooling rate of 10° C./sec or less. Then, the steelsheet may be cooled and controlled to an appropriate temperature throughforced cooling with spraying of a coolant, such as water, air blowing,or mist or the like, and over-aging or tempering is additionally carriedout during the cooling as required. At a temperature of lower than 600°C., the fraction of bainite is insufficient and the ductility isdecreased. At a temperature of higher than 720° C., the fraction ofbainite is excessive, and the ductility is decreased. In addition, whenthe cooling rate is more than 10° C./sec, the precipitation of ferriteis small and the fraction of bainite becomes excessive, and thus, theductility is decreased. The lower limit of the cooling rate is notparticularly limited, but is preferably set to 1° C./sec or more in viewof the productivity and the cooling controllability.

When hot dip galvanizing or galvannealing after the cooling after theannealing is carried out, the composition of zinc plating is notparticularly limited, and in addition to Zn, Fe, Al, Mn, Cr, Mg, Pb, Sn,Ni, and the like may be added as required. The plating may be carriedout as a separate process from annealing, but is preferably carried outthrough a continuous annealing-hot dip galvanizing line in whichannealing, cooling and plating are continuously carried out in view ofthe productivity. When the following alloying treatment is not carriedout, the steel sheet is cooled to a normal temperature after theplating.

When an alloying treatment is carried out, it is preferable that thealloying treatment be carried out within a temperature range of 450° C.to 600° C. after the plating, and then, the steel sheet be cooled to anormal temperature. This is because alloying does not sufficientlyproceed at a temperature of lower than 450° C., and alloying excessivelyproceeds at a temperature of higher than 600° C. such that the platedlayer is embrittled to cause a problem of exfoliation of the plating byworking such as pressing or the like. When an alloying treatment time isshorter than 10 seconds, alloying does not sufficiently proceed, andthus, 10 seconds or longer is preferable. In addition, the upper limitof the alloying treatment time is not particularly limited, butpreferably within 100 seconds in view of productivity.

In view of productivity, it is preferable that an alloying treatmentfurnace be provided continuously to the continuous annealing-hot dipgalvanizing line to carry out annealing, cooling, plating and analloying treatment, and cooling in a continuous manner.

Examples of the plated layer shown in examples include hot dipgalvanizing and galvannealing, and electrogalvanizing is also included.

Skin pass rolling is carried out to correct the shape and secure thesurface properties, and is preferably carried out in a range of anelongation ratio of 0.2% to 2.0%. The reason why the lower limit of theelongation ratio of the skin pass rolling is set to 0.2% is thatsufficient improvement in the surface roughness is not attained at anelongation ratio of less than 0.2%, and thus, the lower limit is set to0.2%. On the other hand, when the skin pass rolling is carried out atthe elongation ratio of more than 2.0%, the steel sheet is excessivelywork-hardened to deteriorate the press formability. Thus, the upperlimit is set to 2.0%.

Next, a metallographic structure will be described.

The microstructure of the steel sheet obtained by the present inventionis composed of mainly ferrite and bainite. When the area ratio offerrite is less than 80%, bainite is increased and sufficient ductilitycannot be obtained. Thus, the lower limit of the area ratio of ferriteis set to 80%. When the area ratio of ferrite is 95% or more, a tensilestrength of 590 MPa or more cannot be secured in some cases, and thus,the upper limit of the area ratio of ferrite is set to less than 95%.Further, the area ratio of ferrite is preferably 90% or less.

Since the non-recrystallization ferrite contributes to hardening of theferrite, the effect of improving the stretch flangeability is obtainedby reducing the hardness difference with the bainite with appropriatelycontrolling the area ratio of the non-recrystallization ferrite within arange of 1% or more and less than 10%. When the ratio of thenon-recrystallization ferrite in the ferrite is less than 1%, thenon-recrystallization ferrite does not contribute to hardening of theferrite, and thus, the lower limit of the area ratio of thenon-recrystallization ferrite is set to 1% or more. When the ratio ofthe non-recrystallization ferrite in the ferrite is 10% or more, adecrease in the hole expansion ratio or the like is caused, and thus,the upper limit is set to less than 10%.

Bainite contributes to high-strengthening. However, when the amount ofbainite is excessive, a decrease in the ductility is caused, and thus,the lower limit is set to 5% and the upper limit is set to 20%.

In addition, as shown in FIG. 7, as other phase, there are pearlite,residual austenite, and martensite. When a total amount (area ratio orvolume ratio) of these components is 8% or more, the hardness differencewith the ferrite is large, and thus, the hole expansion ratio or thelike is decreased. Therefore, the upper limit of the total amount of thepearlite, residual austenite, and martensite is set to less than 8%.When the structure of the present invention can be obtained in thecomponent range of the present invention, a tensile strength of 590 MPaor more can be obtained. The upper limit of the tensile strength is notparticularly limited. However, considering the lower limit of the arearatio of the ferrite of the present invention, the upper limit may beset to about 780 MPa.

The equivalent circle diameter of the carbonitrides including one orboth of Nb and Ti is set to 10 nm or less. As shown in FIG. 6, theaverage particle diameter of the carbonitrides is extremely important toappropriately control the amount of the non-recrystallization ferrite,and when the equivalent circle diameter is more than 10 nm, a sufficientrecrystallization suppressing effect cannot be obtained and anappropriate amount of the non-recrystallization ferrite cannot beobtained. Thus, the upper limit is set to 10 nm. In addition, the lowerlimit is set to 1 nm or more in terms of accuracy in measurement.

The microstructure may be observed with an optical microscope bycollecting a sample having an observation surface which is a crosssection parallel to the rolling direction and the thickness direction,polishing the observation surface, and carrying out nital etching, andas required, Le Pera etching. In the observation of the microstructure,regarding the sample collected from an arbitrary position of the steelsheet, a portion which is at a ¼ portion along the thickness directionwas imaged at a magnification of 1000 times in a range of 300×300 μm.The image of the microstructure obtained by the optical microscope isanalyzed by binarizing the image to white and black so that a total arearatio of any one or two or more of pearlite, bainite, and martensite canbe obtained as an area ratio of phases other than the ferrite. Thefraction was measured for the sample collected from an arbitraryposition of the steel sheet with by the above method imaging a ¼ portionalong the thickness direction at a magnification of 1000 times in arange of 300×300 μm and having 3 or more imaged view fields. It isdifficult to distinguish residual austenite from martensite with theoptical microscope, but the volume ratio of the residual austenite canbe measured by an X-ray diffraction method. The sample used in theaforementioned microstructure observation is used for obtaining thefraction of the residual austenite. The non-recrystallization ferriteand ferrite other than the non-recrystallization ferrite can bedetermined by analyzing the measurement data of the orientation of anelectron back scattering pattern (EBSP) by the Kernel AverageMisorientation method (KAM method). In the grains of thenon-recrystallization ferrite, dislocations are recovered, but acontinuous change of the orientation, which is caused by plasticdeformation during the cold rolling, is present. On the other hand, thechange in the orientation in the grains of the ferrite other than thenon-recrystallization ferrite becomes extremely small. In the KAMmethod, it is possible to quantitatively indicate the orientationdifference with an adjacent measurement point. In the present invention,when an area between measurement points, the measurement points having5° or more of average orientation difference, is defined as grainboundary, a grain in which the average orientation difference with anadjacent measurement point is 1° or less and of which the grain size ismore than 0.5 μm is defined as the ferrite other than thenon-recrystallization ferrite. That is, the area ratio of thenon-recrystallization ferrite is an area ratio obtained by subtractingthe area ratio of ferrite other than the non-recrystallization ferritefrom the area ratio of total ferrite. The area ratio obtained from themicrostructure is the same as the volume ratio.

The average particle size of the carbonitrides including one or both ofNb and Ti is measured by preparing an extraction replica sampleextracted from a portion which is at a depth of ¼ of the sheet thicknessfrom a surface of arbitrary position of the steel sheet, and observingcarbonitrides as a target with a transmission type electron microscope(TEM) to obtain the average particle size of the carbonitrides. Theaverage particle size was obtained by imaging an image at amagnification of 10000 times in a range of 10×10 μm, and counting 100random particles of alloy carbides. It is difficult to count a particlehaving a size of 1 nm, and 100 large particles are counted, not exactlyin descending order, but in random order.

A test method of each mechanical property will be described below. Atensile test sample according to JIS Z 2201 No. 5 was taken from a steelsheet after being manufactured in which the width direction (referred toas the TD direction) is considered as the longitudinal direction, andthe tensile properties in the TD direction were evaluated according toJIS Z 2241. The stretch flangeability was evaluated according to JapanIron and Steel Federation Standard JFS T 1001. Each of the obtainedsteel sheets was cut to 100 mm×100 mm size pieces and then punched tohave a hole with a diameter of 10 mm with a clearance being 12% of thethickness. Then, in a state in which blank holding pad were suppressedwith a force of 88.2 kN and in which a die with an inner diameter of 75mm is used, a 60° conical punch was forced through the hole to measure ahole diameter in a crack initiation limit. A limiting hole expansionratio [%] was obtained from the following (Expression 3), and thestretch flangeability was evaluated based on the limiting hole expansionratio.

Limiting hole expansion ratio λ [%]={(D_(f)−D₀)/D₀}×100  (Expression 3)

Here, D_(f) represents a hole diameter [mm] at the time of crackinitiation, and D₀ represents an initial hole diameter [mm]. Inaddition, plating adhesion is evaluated according to JIS H 0401 byvisually observing a surface state of a plating film at a portion bentby a bending test.

Examples

Steel sheets were obtained by melting the steels having the compositionsshown in Table 1, casting to obtain the slabs, and manufacturing thesteel sheetsunder the conditions shown in Tables 2-1 and 2-2. “[-]” inTable 1 indicates that the analyzed value of a component is less than adetection limit. In addition, calculation values of Ar₃ [° C.] and Ar₁[° C.] are also shown in Table 1.

A tensile test sample according to JIS Z 2201 No. 5 was taken from asteel sheet after being manufactured in which the width direction(referred to as the TD direction) is considered as the longitudinaldirection, and the tensile properties in the TD direction were evaluatedaccording to JIS Z 2241. In addition, the stretch flangeability wasevaluated according to Japan Iron and Steel Federation Standard JFS T1001. Each of the obtained steel sheets was cut to 100 mm×100 mm sizepieces and then punched to have a hole with a diameter of 10 mm with aclearance being 12% of the thickness. Then, in a state in which blankholding pad were suppressed with a force of 88.2 kN and in which a diewith an inner diameter of 75 mm is used, a 60° conical punch was forcedthrough the hole to measure a hole diameter in a crack initiation limit.A limiting hole expansion ratio [%] was obtained from the following(Expression 3), and the stretch flangeability was evaluated based on thelimiting hole expansion ratio.

Limiting hole expansion ratio λ [%]={(D_(f)−D₀)/D₀}×100  (Expression 3)

Here, D_(f) represents a hole diameter [mm] at the time of crackinitiation, and D₀ represents an initial hole diameter. In addition,plating adhesion is evaluated according to JIS H 0401 by visuallyobserving a surface state of a plating film at a portion bent by abending test.

The microstructure of the sheet thickness cross section of the steelsheet was observed by the above-described manner, and the area ratio ofbainite was obtained as a total area ratio of phase which is not ferriteand other phases.

The result is shown in Tables 3-1 and 3-2. In the present invention, asample having 17000 [MPa·%] or more of a product of tensile strength TS[MPa] and total elongation El [%], i.e. TS×El [MPa·%], which is aductility index, is considered as acceptance. A sample having 75% ormore of, and preferably 80% or more of the hole expansion ratio λ [%],which is a hole expansibility index, is considered as acceptance. In acase of a hot-dip galvanized steel sheet or a galvannealed steel sheet,plating adhesion is also set as a target. The plating adhesion wasevaluated according to JIS H 0401 by visually observing a surface stateof a plating film at a portion bent by a bending test.

As shown in Tables 3-1 and 3-2, it is possible to obtain a high-strengthsteel sheet, a hot-dip galvanized steel sheet, and a galvannealed steelsheet which have excellent ductility and stretch flangeability bysubjecting steel having the chemical composition of the presentinvention to hot rolling, cold rolling, and annealing under appropriateconditions.

On the other hand, for Steel No. M, since the amount of C is large, thetotal elongation is decreased, the product of tensile strength and totalelongation is decreased, and the hole expansion ratio is also decreased.

For Steel No. N, since the amount of C is small, the area ratio ofbainite is reduced, the tensile strength is decreased, and the productof tensile strength and total elongation is decreased.

For Steel No. O, since the amount of Si is small, the hole expansionratio is decreased.

For Steel No. P, since the amount of Si is large, the area ratio ofbainite is reduced, the tensile strength and the total elongation aredecreased, the product of tensile strength and total elongation isdecreased, and the plating adhesion is also decreased.

For Steel No. Q, since the amount of Mn is small, the area ratio ofbainite is reduced, the tensile strength and the total elongation aredecreased, the product of tensile strength and total elongation isdecreased, and the hole expansion ratio is also decreased.

For Steel No. R, since the amount of Mn is large, the area ratio ofbainite is increased, the tensile strength is increased, the totalelongation is decreased, the product of tensile strength and totalelongation is decreased, and the hole expansion ratio is also decreased.

For Steel No. S, since the amount of Al is large, the area ratio ofbainite is reduced, the tensile strength is decreased, the product oftensile strength and total elongation is decreased, and the holeexpansion ratio is also decreased.

For Steel No. T, since the amount of N is large, the area ratio ofbainite is increased, the total elongation is decreased, the product oftensile strength and total elongation is decreased, and the holeexpansibility is also decreased.

For Steel No. U, since the amount of Ti and Nb is small, the area ratioof non-recrystallization ferrite is reduced, the tensile strength andthe hole expansion ratio are decreased.

For Steel No. V, since the amount of Ti and Nb is large, the area ratioof non-recrystallization ferrite is increased, the total elongation isdecreased, the product of tensile strength and total elongation isdecreased, and the hole expansion ratio is also decreased.

For Steel No. W, since the amount of Nb is small, the area ratio ofnon-recrystallization ferrite is reduced, the tensile strength and thehole expansion ratio are decreased.

For Steel No. X, since the amount of Ti is large, the area ratio ofnon-recrystallization ferrite is increased, the total elongation isdecreased and the product of the tensile strength and the totalelongation is decreased. Also, the hole expansion ratio is decreased.

For Steel No. Y, since the amount of Nb is large, the area ratio ofnon-recrystallization ferrite is increased, the total elongation isdecreased, the product of tensile strength and total elongation isdecreased, and the hole expansion ratio is also decreased.

For Production No. 3, since the heating temperature during the hotrolling is low, the carbonitrides are coarsened and therecrystallization suppressing effect during the annealing is small, andthus, the area ratio of the non-recrystallization ferrite is reduced andthe tensile strength and the hole expansion ratio are decreased.

For Production No. 6, since the finishing temperature during the hotrolling is slightly low, the carbonitrides are coarsened and therecrystallization suppressing effect during the annealing is small, andthus, the area ratio of non-recrystallization ferrite is reduced and thetensile strength and the hole expansion ratio are decreased.

For Production No. 9, since the finishing temperature during the hotrolling is slightly low, the carbonitrides are coarsened and therecrystallization suppressing effect during the annealing is small, andthus, the area ratio of non-recrystallization ferrite is reduced and thetensile strength and the hole expansion ratio are decreased.

For Production No. 12, since the finishing temperature during the hotrolling is low, the carbonitrides are coarsened and therecrystallization suppressing effect during the annealing is small, andthus, the area ratio of non-recrystallization ferrite is reduced and thetensile strength and the hole expansion ratio are decreased.

For Production No. 15, since the coiling temperature is high, thecarbonitrides are coarsened and the recrystallization suppressing effectduring the annealing is small, and thus, the area ratio ofnon-recrystallization ferrite is reduced and the tensile strength andthe hole expansion ratio are decreased.

For Production No. 18, since the cold rolling reduction is low, the arearatio of non-recrystallization ferrite is increased and the totalelongation is decreased, and thus, the product of tensile strength andtotal elongation is decreased, and the hole expansion ratio is alsodecreased.

For Production No. 21, since the maximum heating temperature is highduring the annealing, the carbonitrides are coarsened, and therecrystallization suppressing effect during the annealing is small, thearea ratio of non-recrystallization ferrite is reduced. The area ratioof bainite is increased, and thus, the hole expansion ratio isdecreased.

For Production No. 24, since the maximum heating temperature during theannealing is low, the area ratio of bainite is reduced, and thus, thetensile strength and the total elongation are decreased, and the productof tensile strength and total elongation is decreased. Also, the holeexpansion ratio is decreased.

For Production No. 25, since the end temperature of primary coolingafter annealing is excessively high, the area ratio of ferrite does notreach a predetermined value, and relatively, the area ratio of bainiteis increased. The hole expansion ratio is decreased.

For Production No. 28, the holding time at the maximum heatingtemperature during the annealing is short, the amount of bainite isreduced and the area ratio of non-recrystallization ferrite isincreased. Thus, the total elongation is decreased, the product oftensile strength and total elongation is decreased and the holeexpansion ratio is also decreased.

For Production No. 29, since the end temperature of primary coolingafter annealing is excessively low, the area ratio of ferrite isincreased excessively, and relatively, the area ratio of bainite isreduced excessively. While the hole expansion ratio is satisfied, thetensile strength does not reach a predetermined value, and also thebalance between tensile strength and total elongation is poor. Theproduct of tensile strength and total elongation is also decreased.

For Production No. 32, since the holding time at the maximum heatingtemperature during the annealing is long, the carbonitrides arecoarsened and the recrystallization suppressing effect during theannealing is small, and thus, the area ratio of non-recrystallizationferrite is reduced and the area ratio of bainite is increased. Thus, thehole expansion ratio is decreased.

For Production No. 33, since the annealing primary cooling rate isexcessively high, the area ratio of ferrite does not reach apredetermined value and relatively, the area ratio of bainite isincreased. Thus, the hole expansion ratio is decreased and also thebalance between tensile strength and total elongation is poor. Theproduct of tensile strength and total elongation is also decreased.

For Production No. 34, since the coiling temperature is low and therecrystallization suppressing effect by carbonitrides during theannealing is large, the area ratio of non-recrystallization ferrite isincreased and the total elongation is decreased. The product of tensilestrength and total elongation is decreased and the hole expansion ratiois also decreased.

For Production No. 39, since the temperature rising rate during theannealing is high, the area ratio of non-recrystallization ferrite isincreased and the total elongation is decreased. The product of tensilestrength and total elongation is decreased and the hole expansion ratiois also decreased.

[Table 1]

[Table 2-1]

[Table 2-2]

[Table 3-1]

[Table 3-2]

INDUSTRIAL APPLICABILITY

According to the present invention, it is possible to provide ahigh-strength cold-rolled steel sheet and a plated steel sheet whichhave a tensile strength of 590 MPa or more, and excellent ductility andstretch flangeability, and the present invention makes an extremelysignificant contribution to the industry.

TABLE 1 Ti + STEEL C Si Mn P S Al N Ti Nb Nb Mo No. % % % % % % % % % %% A 0.045 0.45 2.10 0.0071 0.0025 0.030 0.0033 — 0.015 0.015 — B 0.0500.30 2.00 0.0068 0.0028 0.033 0.0034 0.020 — 0.020 — C 0.055 0.25 1.950.0070 0.0027 0.032 0.0033 — 0.025 0.025 — D 0.035 0.70 1.90 0.00690.0026 0.025 0.0039 — 0.020 0.020 — E 0.050 0.30 2.00 0.0067 0.00250.027 0.0035 0.010 0.010 0.020 0.15 F 0.055 0.40 1.85 0.0073 0.00300.028 0.0040 0.025 — 0.025 — G 0.070 0.25 1.50 0.0070 0.0029 0.0330.0035 0.015 0.010 0.025 — H 0.060 0.35 1.95 0.0071 0.0028 0.039 0.0036— 0.010 0.010 0.10 I 0.040 0.50 2.05 0.0065 0.0024 0.035 0.0041 0.0100.015 0.025 — J 0.030 0.65 1.80 0.0073 0.0023 0.031 0.0039 — 0.010 0.0100.15 K 0.065 0.55 1.40 0.0069 0.0024 0.035 0.0033 0.020 — 0.020 0.10 L0.075 0.80 1.20 0.0081 0.0030 0.029 0.0034 — 0.025 0.025 — M 0.150 0.302.00 0.0079 0.0027 0.034 0.0033 — 0.015 0.015 — N 0.010 0.35 1.70 0.00680.0033 0.037 0.0031 0.010 0.010 0.020 — O 0.050 0.05 1.80 0.0077 0.00250.036 0.0038 0.020 — 0.020 — P 0.040 1.50 2.10 0.0070 0.0032 0.0290.0032 — 0.010 0.010 — Q 0.045 0.45 0.50 0.0088 0.0025 0.030 0.0037 —0.020 0.020 0.15 R 0.055 0.30 3.00 0.0071 0.0028 0.031 0.0033 0.015 —0.015 — S 0.035 0.40 2.20 0.0073 0.0031 0.120 0.0034 0.015 0.010 0.025 —T 0.050 0.45 1.90 0.0081 0.0030 0.032 0.0140 0.010 0.015 0.025 — U 0.0500.35 1.95 0.0085 0.0026 0.034 0.0029 0.002 — 0.002 — V 0.045 0.50 2.000.0075 0.0028 0.040 0.0040 0.010 0.025 0.035 — W 0.045 0.40 1.90 0.00780.0032 0.033 0.0041 — 0.002 0.002 — X 0.055 0.45 2.00 0.0080 0.00290.031 0.0035 0.035 — 0.035 — Y 0.050 0.50 1.85 0.0068 0.0031 0.0380.0036 — 0.035 0.035 — STEEL No. W V B Ni Cu Cr Ar₃ Ac₁ A % % % % % % °C. ° C. REMARKS — — — — — — 720 723 INVENTION B STEEL — — — — — — 723721 INVENTION C STEEL — — — — — — 724 721 INVENTION D STEEL — — — 0.30 —— 736 730 INVENTION E STEEL — — — — — 0.15 702 721 INVENTION F STEEL0.20 0.20 — — — — 738 727 INVENTION G STEEL — — 0.0010 — 0.50 — 715 729INVENTION H STEEL — — — — — — 717 725 INVENTION I STEEL 0.10 — 0.00100.15 — — 721 724 INVENTION J STEEL — 0.30 — — 0.30 — 718 729 INVENTION KSTEEL 0.20 — — 0.30 — 0.30 745 738 INVENTION L STEEL — — 0.0015 — 0.500.50 736 749 INVENTION M STEEL — — — — — — 691 742 COM- PARATIVE N STEEL— — — — — — 765 717 COM- PARATIVE O STEEL — — 0.0015 — — — 733 716 COM-PARATIVE P STEEL — — — — — — 756 753 COM- PARATIVE Q STEEL — — — — — —854 740 COM- PARATIVE R STEEL — — — — — — 629 711 COM- PARATIVE S STEEL— — — — — — 716 719 COM- PARATIVE T STEEL — — — — — — 737 726 COM-PARATIVE U STEEL — — — — — — 730 723 COM- PARATIVE V STEEL — — — — —0.15 725 726 COM- PARATIVE W STEEL — — — — — — 737 724 COM- PARATIVE XSTEEL — — — — — — 727 726 COM- PARATIVE Y STEEL — — — — — — 744 728 COM-PARATIVE STEEL

TABLE 2-1 ANNEALING HOT ROLLING COLD TEMPER- MAXIMUM HEATING FINISHINGCOILING ROLLING ATURE HEATING MANU- TEMPER- TEMPER- TEMPER- REDUC-RISING TEMPER- STEEL FACTURING ATURE ATURE ATURE TION RATE ATURE No. No.° C. ° C. ° C. % ° C./sec ° C. A 1 1220 930 580 60 3 750 2 1200 910 55060 3 755 3 1060 920 540 60 2 750 B 4 1200 920 550 65 3 755 5 1220 920560 65 3 755 6 1220 900 550 85 4 760 C 7 1200 940 480 60 3 760 8 1200930 520 60 3 760 9 1210 900 580 65 1 755 D 10 1230 900 550 55 3 760 111250 940 500 60 3 750 12 1210 720 590 60 3 760 E 13 1180 920 570 65 4755 14 1200 930 550 60 3 750 15 1220 920 700 65 3 750 F 16 1220 880 45055 3 765 17 1190 900 550 60 3 760 18 1200 920 300 25 3 755 G 19 1240 920590 60 2 745 20 1220 910 580 60 3 755 21 1200 900 590 65 3 820 H 22 1220900 450 55 3 760 23 1250 900 500 60 4 750 24 1200 910 550 60 3 600 251210 900 550 55 4 750 ANNEALING SKIN PASS END ROLLING TEMPER- ALLOYINGAFTER ATURE COOLING TREAT- ALLOYING ANNEALING HOLD- OF RATE OF MENTTREAT- ELONG- ING PRIMARY PRIMARY TEMPER- MENT ATION STEEL TIME COOLINGCOOLING APTURE TIME RATIO No. sec ° C. ° C./sec ° C. sec % A 100 620 3540 25 0.4 80 680 5 — — 0.6 120 650 4 550 30 0.4 B 100 630 3 550 30 0.6120 700 3 540 30 0.6 100 650 6 550 20 0.6 C 80 640 5 550 25 0.6 100 6904 550 30 0.4 120 620 3 540 30 0.6 D 80 650 5 — — 0.6 100 630 5 550 300.8 100 670 5 550 25 0.8 E 100 710 3 — — 0.6 100 660 2 520 30 0.6 120650 3 540 30 0.8 F 80 650 2 550 20 0.7 100 680 2 — — 0.4 80 670 3 530 350.5 G 100 640 5 550 30 0.5 100 630 4 560 30 0.8 120 610 4 550 35 0.8 H100 700 3 — — 0.8 80 680 5 — — 0.8 100 680 3 570 20 0.6 90 750 3 540 250.4 (ANNOTATION 1) THE UNDERLINED VALUES ARE OUT OF THE RANGE OF THEPRESENT INVENTION

TABLE 2-2 ANNEALING HOT ROLLING COLD TEMPER- MAXIMUM HEATING FINISHINGCOILING ROLLING ATURE HEATING MANU- TEMPER- TEMPER- TEMPER- REDUC-RISING TEMPER- STEEL FACTURING ATURE ATURE ATURE TION RATE ATURE No. No.° C. ° C. ° C. % ° C./sec ° C. I 26 1200 920 550 60  4 755 27 1190 920550 65  3 755 28 1220 900 550 55  3 750 29 1210 910 540 60  3 760 J 301220 900 480 60  3 765 31 1230 920 590 65  2 760 32 1220 900 520 50  3755 33 1200 900 530 60  3 760 K 34 1220 900 400 55  2 770 35 1210 890550 60  2 770 36 1220 900 530 60  3 760 L 37 1210 920 550 65  3 770 381220 910 600 60  3 780 39 1240 900 450 65 10 765 M 40 1220 920 550 60  3770 N 41 1200 900 550 55  3 750 O 42 1200 930 600 60  3 750 P 43 1220900 580 65  3 770 Q 44 1230 900 450 60  3 770 R 45 1220 920 530 55  3750 S 46 1220 900 550 65  3 750 T 47 1200 900 550 60  3 760 U 48 1190900 530 60  3 760 V 49 1180 920 570 60  3 760 W 50 1200 920 580 60  3760 X 51 1220 900 530 65  3 760 Y 52 1200 900 550 60  3 760 ANNEALINGSKIN PASS END ROLLING TEMPER- ALLOYING AFTER ATURE COOLING TREAT-ALLOYING ANNEALING HOLD- OF RATE OF MENT TREAT- ELONG- ING PRIMARYPRIMARY TEMPER- MENT ATION STEEL TIME COOLING COOLING APTURE TIME RATIONo. sec ° C. ° C./sec ° C. sec % I 100 660  3 530 30 0.6 120 640  2 — —0.6  2 670  3 550 30 0.4 120 570  7 540 30 0.6 J 100 650  8 — — 0.8 100700  5 540 25 0.8 400 710  4 530 35 0.6  80 670 15 550 30 0.6 K 100 630 3 — — 0.6  80 620  2 540 30 0.6 100 630  3 550 30 0.4 L 120 630  3 52035 0.4 120 650  3 — — 0.6 100 630  2 550 20 0.4 M 120 690  4 550 30 0.4N 100 700  5 540 25 0.6 O  80 680  5 550 30 0.8 P 100 670  4 540 30 0.6Q 100 630  4 — — 0.4 R 120 640  4 550 30 0.6 S  80 650  5 540 30 0.6 T120 650  3 540 30 0.8 U 100 650  2 550 30 0.6 V 120 660  4 550 25 0.6 W 80 630  3 — — 0.6 X 120 620  5 540 30 0.8 Y 100 680  2 530 35 0.6(ANNOTATION 1) THE UNDERLINED VALUES ARE OUT OF THE RANGE OF THE PRESENTINVENTION

TABLE 3-1 MICROSTRUCTURE AREA RATIO TOTAL CARBO- MECHANICAL AREA OF NON-AREA AMOUNT NITRIDE PROPERTIES RATIO RECRYSTAL- RATIO OF THE EQUIVALENTYIELD MANU- OF LIZATION OF OTHER CIRCLE STRENGTH STEEL FACTURING FERRITEFERRITE BAINITE PHASE DIAMETER YP No. No. % % % % nm MPa A 1 89 7 11 0 7440 2 87 6 13 0 8 450 3 88 0 12 0 30  400 B 4 88 8 11 1 6 445 5 87 7 130 7 455 6 87 0 13 0 25  405 C 7 89 9 10 1 5 450 8 88 8 11 1 6 460 9 88 012 0 35  400 D 10 88 9 12 0 5 445 11 89 9 11 0 5 445 12 86 0 14 0 25 405 E 13 88 7 12 0 7 420 14 89 8 11 0 6 420 15 88 0 11 1 15  410 F 16 877 13 0 7 470 17 88 8 12 0 6 470 18 88 20  12 0 4 510 G 19 86 9 12 2 4480 20 90 8 10 0 6 440 21 75 2 15 10  25  390 H 22 88 5 10 2 9 440 23 876 12 1 8 440 24 100  50   0 0 5 480 25 72 5 25 3 5 380 MECHANICALPROPERTIES TOTAL HOLE TENSILE ELONG- EXPAN- STRENGTH ATION SION PLATINGSTEEL TS El TS x El RATIO λ AD- No. MPa % MPa · % % HESION A 610 2917690 100 GOOD 630 28 17640 90 GOOD 580 30 17400 70 GOOD B 620 28 17360105 GOOD 635 28 17780 95 GOOD 580 30 17400 70 GOOD C 620 28 17360 110GOOD 635 28 17780 100 GOOD 580 30 17400 70 GOOD D 625 28 17500 90 — 62028 17360 95 GOOD 585 30 17550 70 GOOD E 615 29 17835 105 GOOD 615 2917835 105 GOOD 580 30 17400 70 GOOD F 630 28 17640 95 GOOD 630 28 1764095 — 660 24 15840 65 GOOD G 640 27 17280 85 GOOD 615 29 17835 100 GOOD710 24 17040 50 GOOD H 630 28 17640 95 — 625 28 17500 100 — 560 26 1456055 GOOD 700 25 17500 65 GOOD (ANNOTATION 1) THE UNDERLINED VALUES AREOUT OF THE RANGE OF THE PRESENT INVENTION

TABLE 3-2 MICROSTRUCTURE AREA RATIO TOTAL CARBO- MECHANICAL AREA OF NON-AREA AMOUNT NITRIDE PROPERTIES RATIO RECRYSTAL- RATIO OF THE EQUIVALENTYIELD MANU- OF LIZATION OF OTHER CIRCLE STRENGTH STEEL FACTURING FERRITEFERRITE BAINITE PHASE DIAMETER YP No. No. % % % % nm MPa I 26 88 9 12 04 445 27 87 9 12 1 5 445 28 97 15   3 0 3 520 29 98 5  2 0 4 340 J 30 876 12 1 8 440 31 88 7 12 0 7 440 32 75 0 22 3 25  400 33 73 5 24 3 5 500K 34 86 15  13 1 2 510 35 87 8 13 0 6 445 36 87 9 13 0 5 460 L 37 88 812 0 6 450 38 86 7 13 1 7 450 39 89 30  11 0 2 580 M 40 88 8 12 0 6 520N 41 100  8  0 0 6 360 O 42 90 9 10 0 4 460 P 43 97 4  3 0 10  500 Q 4498 9  2 0 4 500 R 45 60 7 30 10  7 500 S 46 97 8  3 0 6 500 T 47 70 8 255 6 480 U 48 87 0 12 1 8 360 V 49 87 25  13 0 7 515 W 50 88 0 12 0 8 360X 51 87 20  13 0 7 500 Y 52 87 25  12 1 7 515 MECHANICAL PROPERTIESTOTAL HOLE TENSILE ELONG- EXPAN- STRENGTH ATION SION PLATING STEEL TS ElTS x El RATIO λ AD- No. MPa % MPa · % % HESION I 625 28 17500 105 GOOD625 28 17500 100 GOOD 605 26 15730 70 GOOD 550 30 16500 80 GOOD J 615 2917835 105 — 615 29 17835 105 GOOD 680 26 17680 60 GOOD 720 23 16560 60GOOD K 640 25 16000 70 GOOD 625 28 17500 90 GOOD 630 28 17640 90 GOOD L620 28 17360 100 GOOD 630 28 17640 90 — 655 24 15720 60 GOOD M 720 2215840 50 GOOD N 550 30 16500 95 GOOD O 620 28 17360 60 GOOD P 580 2816240 120 BAD Q 585 28 16380 70 — R 720 22 15840 65 GOOD S 580 28 1624070 GOOD T 680 23 15640 65 GOOD U 585 30 17550 65 GOOD V 640 25 16000 60GOOD W 585 30 17550 65 — X 630 25 15750 60 GOOD Y 640 25 16000 60 GOOD(ANNOTATION 1) THE UNDERLINED VALUES ARE OUT OF THE RANGE OF THE PRESENTINVENTION

1. A cold-rolled steel sheet comprising, by mass %: C: 0.020% or moreand 0.080% or less; Si: 0.20% or more and 1.00% or less; Mn: 0.80% ormore and 2.30% or less; P: 0.0050% or more and 0.1500% or less; S:0.0020% or more and 0.0150% or less; Al: 0.010% or more and 0.100% orless; N: 0.0010% or more and 0.0100% or less; and further comprising:one or more of Nb and Ti which satisfy a requirement of0.005%≤Nb+Ti<0.030%; and a remainder including Fe and unavoidableimpurities, wherein a structure consists of, a ferrite, a bainite, andan other phase, the other phase includes one or more of a pearlite, aresidual austenite, and a martensite, an area ratio of the ferrite is80% or more and less than 95%, an area ratio of a non-recrystallizationferrite in the ferrite is 1% or more and less than 10%, an area ratio ofthe bainite is 5% to 20%, a total amount of the other phase is less than8%, an equivalent circle diameter of a carbonitride including one orboth of Nb and Ti is 1 nm or more and 10 nm or less, and a tensilestrength is 590 MPa or more.
 2. The cold-rolled steel sheet according toclaim 1, further comprising one or more of, by mass %: Mo: 0.005% ormore and 1.000% or less; W: 0.005% or more and 1.000% or less; V: 0.005%or more and 1.000% or less; B: 0.0005% or more and 0.0100% or less; Ni:0.05% or more and 1.50% or less; Cu: 0.05% or more and 1.50% or less;and Cr: 0.05% or more and 1.50% or less.
 3. A plated steel sheet,wherein a plating is provided on a surface of the cold-rolled steelsheet according to claim
 1. 4. A plated steel sheet, wherein a platingis provided on a surface of the cold-rolled steel sheet according toclaim
 2. 5. A cold-rolled steel sheet comprising, by mass %: C: 0.020%or more and 0.080% or less; Si: 0.20% or more and 1.00% or less; Mn:0.80% or more and 2.30% or less; P: 0.0050% or more and 0.1500% or less;S: 0.0020% or more and 0.0150% or less; Al: 0.010% or more and 0.100% orless; N: 0.0010% or more and 0.0100% or less; and further comprising:one or more of Nb and Ti which satisfy a requirement of0.005%≤Nb+Ti<0.030%; and a remainder including Fe and unavoidableimpurities, wherein a structure comprises a ferrite and a bainite andoptionally an other phase, wherein the optional other phase, whenpresent, includes one or more of a pearlite, a residual austenite, and amartensite, said optional other phase, when present, is present in atotal amount of up to less than 8%, an area ratio of the ferrite is 80%or more and less than 95%, an area ratio of a non-recrystallizationferrite in the ferrite is 1% or more and less than 10%, an area ratio ofthe bainite is 5% to 20%, an equivalent circle diameter of acarbonitride including one or both of Nb and Ti is 1 nm or more and 10nm or less, and a tensile strength is 590 MPa or more.
 6. Thecold-rolled steel sheet according to claim 5, further comprising one ormore of, by mass %: Mo: 0.005% or more and 1.000% or less; W: 0.005% ormore and 1.000% or less; V: 0.005% or more and 1.000% or less; B:0.0005% or more and 0.0100% or less; Ni: 0.05% or more and 1.50% orless; Cu: 0.05% or more and 1.50% or less; and Cr: 0.05% or more and1.50% or less.
 7. A plated steel sheet, wherein a plating is provided ona surface of the cold-rolled steel sheet according to claim
 5. 8. Aplated steel sheet, wherein a plating is provided on a surface of thecold-rolled steel sheet according to claim 6.